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how to find the phase of the signal in fft??

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sandy3129

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hello guys, im implementing a fft of frequency 4ghz sampled at a frequency of 10ghz, given a phase angle of "0" degree to real and "(pi/2)" degree to imag, using 512 point fft in matlab system generator,, my index(bin number) is proper i.e the highest peak is (@205) bin , so im able to find out the input frequency , but the phase angle is showing as 70 degree i did ((tan-1(imag/real))) number, the expected is 0 degree , kindly anyone help me!!!
 

If it is 70 deg const at all frequencies, then it may be (??) some artifact; but if the phase shows variations across the spectrum then you will have to dig (and dig) more (and perhaps more)...
 

it is not constant across all frequencies, i'm checking with same input frequency with the phase changing from 0 to 90 degrees !! , magnitude is matching, phase is not matching?
 

Absolute value of complex FFT values = magnitude (amplitude) spectrum

Argument of complex FFT values = phase spectrum
 

ustinoff said:
Absolute value of complex FFT values = magnitude (amplitude) spectrum

Argument of complex FFT values = phase spectrum
Click to expand...

ustinoff said:
Absolute value of complex FFT values = magnitude (amplitude) spectrum

Argument of complex FFT values = phase spectrum
Click to expand...

true i have gt my magnitude spectrum, my phase is not proper, i have taken an experiment for that,
i have given 1ghz freq with 0 degree phase for one fft, and i have given the same signal with 90 degrees phase for another fft, the diff in phase is expected to be 90 degree right?? i have got 83, similarly 30 degree phase fft for first one and 90 degree phase fft for second one the diff in phase is 60 right? i have got 64. any suggestions am i missing anything any concept???
 

sandy3129 said:
true i have gt my magnitude spectrum, my phase is not proper, i have taken an experiment for that,
i have given 1ghz freq with 0 degree phase for one fft, and i have given the same signal with 90 degrees phase for another fft, the diff in phase is expected to be 90 degree right?? i have got 83, similarly 30 degree phase fft for first one and 90 degree phase fft for second one the diff in phase is 60 right? i have got 64. any suggestions am i missing anything any concept???
Click to expand...

Check what are using - degrees or radians. As i understood you are using Matlab; Matlab use radians by default.
 
Are you analysing a real signal sampled with an ADC, or a simulated one?
If sampling is not synchronized with the signal, then the phase is random.
 
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ustinoff said:
Check what are using - degrees or radians. As i understood you are using Matlab; Matlab use radians by default.
Click to expand...

thanku sir, sorry for the delayed ans, it worked what ever the result i get from the cordic atan , i should make it (pi/180) and then check it , it is working.
 

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